This invention relates to acoustic surface wave devices employing piezoelectric crystal substrates.
Acoustic delay lines, particularly suitable for microwave frequencies and utilizing surface waves on the face of a piezoelectric crystal substrate are known. In such delay lines, the surface waves may be launched by an interdigital type of input transducer having electrode fingers affixed to the propagation surface by standard photolithographic techniques. According to one such known device described in Electronic Letters, Volume 8, Number 6, published Aug. 10, 1972 the surface waves travel over the propagation surface through a grating consisting of numerous parallel arranged electrodes spaced from one another by grooves the spacings of which increase as a function of the distance from the interdigital electrode input transducer.
I have observed that there is an unexpected beating of a surface acoustic wave signal with a spurious signal believed to be caused by the bulk shear wave generated by interaction of the surface wave with a metal electrode pattern. FIG. 1 shows how a surface acoustic wave is incident on an interdigital electrode pattern common to surface acoustic wave transducers, and how a scattered shear wave is generated in a direction normal (at right angles) to the plane of the surface along which the launched or transmitted surface waves travel for a particular surface electrode or finger spacing.
A problem with the reflective array compressor (RAC) described and illustrated in the Electronic Letters article is that the acoustic beam is critically controlled by the orientation of the scatterers (etched grooves) which scatter energy in the plane of the surface wave. These electrode fingers in the arrays must be at about 45.degree. with respect to the beam direction. Misorientation in the prior art with respect to the crystal axis requires angle compensation in the form of metal film wedges. This last compensation is an involved and tedious process. Also, the scattering angle is temperature dependent, which makes the opeeration vulnerable to temperature changes.
The delay line of the present invention does not suffer from the foregoing problem because the waves are guided by the planar surface, and oppositely disposed surfaces of the crystal substrate of the invention are used to form the legs which the beams must traverse and the scatterers (electrodes) must also be on these same surfaces. The device of the invention has the additional advantage of requiring only half of the volume of the piezoelectric substrate because both oppositely disposed surfaces or faces of the substrate are used. Because the delay of the invention scatters energy at right angles to the plane of the surface wave and through the thickness of the piezoelectric crystal substrate, there is an ease of fabrication and operation which is lacking in the foregoing known type of reflective array compressor.